U.S. patent number 5,369,289 [Application Number 07/969,769] was granted by the patent office on 1994-11-29 for gallium nitride-based compound semiconductor light-emitting device and method for making the same.
This patent grant is currently assigned to Kabushiki Kaisha Toyota Chuo Kenkyusho, Toyoda Gosei Co. Ltd.. Invention is credited to Takahiro Kozawa, Makoto Tamaki.
United States Patent |
5,369,289 |
Tamaki , et al. |
November 29, 1994 |
Gallium nitride-based compound semiconductor light-emitting device
and method for making the same
Abstract
A light-emitting device comprises an n-type layer made of an
n-type gallium nitride-based compound of the formula Al.sub.x
Ga.sub.1-x N, wherein 0.ltoreq.X<1, and an i-type layer formed
on the n-type layer and made of a semi-insulating i-type gallium
nitride-based compound semiconductor and doped with a p-type
impurity for junction with the n-type layer. A first electrode is
formed on the surface of the i-type layer and made of a transparent
conductive film and a second electrode is formed to connect to the
n-type layer through the i-type layer. The device is so arranged
that light is emitted from the side of the i-type layer to the
outside. When an electric current is supplied to the first
electrode from a wire contacted thereto, the first electrode is
held entirely at a uniform potential. Light is emitted from the
entire interface beneath the first electrode and can thus be picked
up from the first electrode which is optically transparent.
Inventors: |
Tamaki; Makoto (Inazawa,
JP), Kozawa; Takahiro (Owariasahi, JP) |
Assignee: |
Toyoda Gosei Co. Ltd.
(Nishikasugai, JP)
Kabushiki Kaisha Toyota Chuo Kenkyusho (Aichi,
JP)
|
Family
ID: |
18047759 |
Appl.
No.: |
07/969,769 |
Filed: |
October 30, 1992 |
Foreign Application Priority Data
|
|
|
|
|
Oct 30, 1991 [JP] |
|
|
3-313977 |
|
Current U.S.
Class: |
257/99; 257/103;
257/749; 257/766; 257/94 |
Current CPC
Class: |
H01L
33/32 (20130101); H01L 33/382 (20130101); H01L
24/06 (20130101); H01L 33/40 (20130101); H01L
2224/45144 (20130101); H01L 24/45 (20130101); H01L
2224/45144 (20130101); H01L 2924/00014 (20130101); H01L
2224/0603 (20130101); H01L 2224/73265 (20130101); H01L
2924/00014 (20130101); H01L 2924/00014 (20130101); H01L
2224/48 (20130101); H01L 2924/12036 (20130101); H01L
2924/12041 (20130101); H01L 2924/12036 (20130101); H01L
2924/00 (20130101); H01L 2924/12041 (20130101); H01L
2924/00 (20130101) |
Current International
Class: |
H01L
33/00 (20060101); H01L 029/205 (); H01L
033/00 () |
Field of
Search: |
;257/94,101,99,102,103,749,766 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Jackson; Jerome
Attorney, Agent or Firm: Cushman, Darby & Cushman
Claims
What is claimed is:
1. A semiconductor light-emitting device which comprises:
an n-type layer made of an n-type gallium nitride-based compound
semiconductor of the formula Al.sub.x Ga.sub.1-x N, wherein
0.ltoreq.X<1;
an i-type layer formed on the n-type layer and made of a
semi-insulating i-type gallium nitride-based compound semiconductor
of the formula Al.sub.x Ga.sub.1-x N, wherein 0.ltoreq.X<1, and
doped with a p-type impurity for junction with the n-type
layer;
a first electrode formed on the surface of the i-type layer and
made of a transparent conductive film; and
a second electrode formed to connect to the n-type layer wherein
light is emitted from the side of the i-type layer to the
outside,
wherein said first electrode is formed at a central portion of said
i-type layer and said second electrode is provided around said
first electrode and connected to a side wall of said n-type
layer.
2. A semiconductor light-emitting device according to claim 1,
wherein said second electrode is formed to connect with said n-type
layer by making cross-cut grooves from a side of said i-type layer
to the surface of said sapphire substrate depending on the chip
size of a light-emitting device, and a metal material, filling said
grooves and separating said sapphire substrate along said cross-cut
grooves.
3. A semiconductor light-emitting device according to claim 1,
further comprising a sapphire substrate on which said n-type layer
is formed, said sapphire substrate having a reflection layer on a
side opposite to said n-type layer.
4. A semiconductor light-emitting device according to claim 3,
further comprising a frame substrate to which said reflection layer
is connected.
5. A semiconductor light-emitting device according to claim 1,
wherein said transparent conductive film consists of tin-added
indium oxide (ITO).
6. A semiconductor light-emitting device according to claim 1,
further comprising a terminal electrode formed at one corner of
said first electrode and having a nickel lowermost layer.
7. A semiconductor light-emitting device according to claim 1,
wherein said second electrode has a three layer structure
consisting of aluminum, nickel and gold layers formed in this order
from a side contacting the n-type layer.
8. A semiconductor light-emitting device according to claim 1,
wherein said n-type layer is of double-layer structure including an
n-layer of low carrier concentration and an n.sup.+ -layer of high
carrier concentration, the former being adjacent to said
i-layer.
9. A semiconductor light-emitting device according to claim 3,
further comprising a buffer layer formed on said sapphire
substrate.
10. A semiconductor light-emitting device which comprises:
an n-type layer made of an n-type gallium nitride-based compound
semiconductor of the formula Al.sub.x Ga.sub.1-x N, wherein
0.ltoreq.X<1;
an i-type layer formed on the n-type layer and made of a
semi-insulating i-type gallium nitride-based compound semiconductor
of the formula Al.sub.x Ga.sub.1-x N, wherein 0.ltoreq.X<1, and
doped with a p-type impurity for junction with the n-type
layer;
a first electrode formed on the surface of the i-type layer and
made of a transparent conductive film; and
a second electrode formed to connect to the n-type layer through
the i-type layer, wherein light is emitted from the side of the
i-type layer to the outside,
wherein said second electrode is connected to said n-type layer at
a central portion of said i-type layer through said i-type layer
and said first electrode is formed on said i-type layer around said
second electrode and in spaced relation to said second
electrode.
11. A semiconductor light-emitting device according to claim 10,
further comprising a sapphire substrate on which said n-type layer
is formed, said sapphire substrate having a reflection layer on a
side opposite to said n-type layer.
12. A semiconductor light-emitting device according to claim 10,
further comprising a reflection film and a frame substrate to which
said reflection film is connected.
13. A semiconductor light-emitting device according to claim 10,
wherein said transparent conductive film of said first electrode
consists of tin-added indium oxide (ITO).
14. A semiconductor light-emitting device according to claim 10,
further comprising a terminal electrode formed at one corner of
said first electrode and having a nickel lowermost layer.
15. A semiconductor light-emitting device according to claim 10,
wherein said second electrode has a three layer structure
consisting of aluminum, nickel and gold layers formed in that order
from a side contacting the n-type layer.
16. A semiconductor light-emitting device according to claim 10,
wherein said second electrode is formed to connect with said n-type
layer by making a hole which extends through a part of said i-type
layer to said n-type layer and filling the hole with a metal
material.
17. A semiconductor light-emitting device according to claim 10,
wherein said n-type layer is of double-layer structure including an
n-layer of low carrier concentration and an n.sup.+ -layer of high
carrier concentration, the former being adjacent to said
i-layer.
18. A semiconductor light-emitting device according to claim 10,
further comprising a buffer layer formed on said sapphire
substrate.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a gallium nitride-based compound
semiconductor light-emitting device which is able to emit blue
light or light in a short wavelength spectral range. The invention
also relates to a method for making the device.
2. Description of Related Art
Light-emitting diodes using GaN-based compound semiconductors
(Al.sub.x Ga.sub.1-x N wherein 0.ltoreq.X<1) are known as ones
which are able to emit blue light or light in a short wavelength
spectral range. Attention has been now directed to the GaN-based
compound semiconductors because they come in direct transition so
that a high light emission efficiency is attained, and are able to
emit blue light which is one of the three primary colors.
With such GaN compound semiconductors, low resistance p-type
crystals are not obtained. In general, a light-emitting diode using
a GaN compound semiconductor is arranged to have a so-called MIS
structure which includes a metal electrode, an i-type layer
(insulator) made of semi-insulating GaN and an n layer made of
n-type GaN. Light emission takes place at a portion beneath the
electrode (light emission electrode) on the i-type layer. More
particularly, the electrode-forming portion has the MIS
structure.
In a GaN blue LED having an MIS structure as mentioned above, it is
important that the device structure and the layer arrangement be
established firsthand in order to have light emitted
efficiently.
In light-emitting devices having a pn junction structure wherein
other compound semiconductors of groups III-V, such as Al.sub.x
Ga.sub.1-x As are used, an electric current is diffused
transversely along the interface of the junction in the device and,
thus, the current passes vertically and uniformly with respect to
the interface. As a consequence, unlike an MIS-type LED wherein
light is emitted only at a portion beneath the electrode, light is
emitted from the entire interface irrespective of the size of the
electrode. Because the light is substantially uniformly emitted
from the interface, pickup of light is easy.
However, with a GaN blue light LED having an MIS structure, little
current diffusion along the transverse direction parallel to the
interface takes place in the i-type layer beneath the
light-emitting electrode. This results in a light-emitting portion
which is limited only to a region beneath the light-emitting
electrode. Because the electrode is generally made of a metal,
light emission is rarely observed from the side of the light
emission electrode as if disappearing behind the electrode.
To avoid this, known GaN blue light LEDs make use of a sapphire
substrate and GaN, both of which are transparent to emission light.
More particularly, it is customary to utilize a flip-chip system
wherein a light emission electrode is provided at the lower side of
the substrate or, instead, is provided in a system wherein light is
picked up from the back side through the substrate. To this end, a
light emission electrode and an electrode electrically connected to
an n-type layer (an electrode at the side of the n-type layer) are
formed on the surface of a GaN epitaxial layer. These electrodes
are bonded with a lead frame by means of a solder, making it
possible to pick up light through the substrate.
However, when using the flip-chip system wherein a light emission
electrode (i-type layer electrode), an n-type layer electrode and a
lead frame are bonded through a solder, the electric series
resistance component of the device has to be increased for the
following reasons:
(1) Because the distance between the electrodes cannot be made too
narrow in order to prevent short-circuiting the light emission
electrode (i-type layer electrode, n-type layer electrode and the
solder), the electric resistance component becomes large.
(2) If the light emission electrode (i-type layer electrode) and
the n-type layer electrode greatly differ in shape under which a
solder bump is formed, the solder bumps have inevitably different
heights, so that a connection failure with the lead frame will be
likely to occur.
Accordingly, it is necessary to shape the electrodes so as to have
substantially the same area. This leads to a loss in the degree of
design freedom of an electrode pattern, further resulting in
difficulty in obtaining an optimum pattern for reducing the
electric resistance component. The large electric series resistance
component not only lowers the light emission efficiency, but also
unfavorably induces generation of heat in the device which causes
device operation to degrade and light emission intensity to become
lower.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a
light-emitting device which is improved in light pickup efficiency
and light emission efficiency while suppressing an electric
resistance component to an extent as low as possible.
It is another object of the invention to provide a method for
making a light-emitting device of the type mentioned above.
The above object can be achieved, according to the invention, by a
gallium nitride-based compound semiconductor light emission device
of the type which comprises an n-type layer made of an n-type
gallium nitride-based compound semiconductor of the formula
Al.sub.x Ga.sub.1-x N wherein 0.ltoreq.X<1, and an i-type layer
formed on the n-type layer and made of a gallium nitride-based,
semi-insulating gallium nitride-based compound semiconductor of the
formula Al.sub.x Ga.sub.1-x N wherein 0.ltoreq.X<1 which is
doped with a p-type impurity for junction with the n-type
layer.
The device also includes a first electrode formed on one side of
the i-type layer and formed of a transparent conductive film, and a
second electrode connected to the n-type layer through the i-type
layer, light being emitted from the i-type layer to the
outside.
In the device of the invention, on the semi-insulating i-type
layer, the first electrode made of a transparent conductive film is
formed. Light is emitted through the first electrode. The light
emission area is defined by the area of the first electrode. The
first electrode is conductive in nature, so that even if an
electric current is only partially supplied to the first electrode,
the first electrode is entirely held at a uniform potential,
thereby causing light to be emitted from the entire lower surface
of the first electrode.
As stated above, the gallium nitride-based compound semiconductor
light emitting device of the invention makes use of a transparent
conductive film as the first electrode (light emission electrode).
Needless to say, the transparent conductive film is transparent to
visible light, making it possible to pick up light from the side of
the light emission electrode. This ensures a number of significant
effects as follows.
1. The electrode can be mounted as an uppermost layer and can be
connected through an ordinary wire bonding method without use of
any solder. If a lead wire is spot connected to the first
electrode, an electric current can be diffused in parallel
directions owing to the conductivity of the first electrode. The
uniform potential of the first electrode is ensured. This would
possibly narrow a wire bonding pad with respect to the first
electrode. This allows the first electrode (light emission
electrode) and the second electrode (n-type electrode) to be kept
at a distance therebetween sufficient to prevent short-circuiting
in device fabrication processes such as photolithography, etching,
lift-off and the like.
In the known flip-chip system, the two electrodes should be kept
away from each other at a distance much longer than a limited
distance created by the lithographic or etching technique so as to
prevent short-circuiting between solders for the two electrodes.
This in turn prevents the area of the first electrode from being
widened.
In the practice of the invention, the ratio of the first electrode
area to the total chip area can be increased, resulting in an
improvement of the light emission efficiency. The distance between
the two electrodes can be made significantly smaller than that
selected in a flip-chip system. This leads to a reduction of the
electric resistance component of the device.
2. Although the flip-chip system requires a first electrode (light
emission electrode) and a second electrode (n-type layer electrode)
which have the same pattern, it is possible in the present
invention to design an optimum pattern for reducing the electric
resistance component of the device owing to an increasing degree of
freedom of design of the two electrode patterns.
3. Because of the small distance between the first electrode (light
emission electrode) and the second electrode (n-type layer
electrode) and the increase in the degree of freedom of design of
the electrode pattern, it is possible to miniaturize the chip size
relative to the light emission area and enlarge the light emission
area, resulting in the economic fabrication of the device.
4. The device of the invention can be assembled into a hybrid unit
with other light emission devices such as AlGaAs red light LED,
within the same lead frame, making it easy to fabricate a light
emission device of multi-colors such as light, green and red
colors.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic sectional view showing a chip structure of a
light-emitting device according to one embodiment of the
invention;
FIG. 2 is a schematic sectional view of a light emitting diode
structure using the chip structure;
FIGS. 3 to 9 are, respectively, a schematic sectional view showing
a fabrication sequence of the light-emitting diode of FIG. 1;
FIG. 10 is a schematic sectional view showing a light-emitting
diode according to another embodiment of the invention;
FIG. 11 is a schematic sectional view showing a light-emitting
diode according to a further embodiment of the invention;
FIGS. 12-15 are, respectively, schematic sectional views of a wafer
during the fabrication process of the light-emitting diode of FIG.
11;
FIG. 16 is a schematic sectional view of a light-emitting diode
according to a still further embodiment of the invention; and
FIG. 17 is a plan view of the light-emitting diode of FIG. 16.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following specific embodiments of the present invention are
described with reference to the accompanying drawings.
FIG. 1 shows a light-emitting diode to which a gallium
nitride-based compound semiconductor device of the invention is
applied.
A light-emitting diode 10 has a sapphire substrate 1 and a 500
angstrom thick AlN buffer layer 2 formed thereon. An approximately
2.5 .mu.m thick n-type layer 4 made of n-type GaN is formed on
buffer layer 2. In addition, an approximately 0.2 .mu.m thick
i-type layer 5 made of semi-insulating GaN is formed on n-type
layer 4. A recess 21 which reaches n-type layer 4 through i-type
layer 5 is also formed. A second electrode 8 made of a metal
material is formed to fill recess 21 for connection with n-type
layer 4.
Further, a first electrode 7 which is kept away from second
electrode 8 is formed on i-type layer 5. First electrode 7 is a
transparent conductive film made of tin-added indium oxide
(hereinafter abbreviated as ITO). First electrode 7 has terminal
electrode 9 formed at a corner portion thereof. Terminal electrode
9 is constituted of two layers including an Ni layer 9a and an Au
layer 9b. The second electrode 8 is constituted of three layers
including an Al layer 8a connected to n-type layer 4, an Ni layer
8b and an Au layer 8c. In this type of light-emitting diode 10,
sapphire substrate 1 has an Al reflection film 13 vacuum deposited
on the opposite side of the sapphire substrate 1.
Light-emitting diode 10 is mounted on a substrate 40 as shown in
FIG. 2 and is electrically connected to lead pins 41, 42 provided
vertically to the substrate 40. More particularly, the Au layer 9b
of the terminal electrode 9 connected to the first electrode 7 is
connected to the lead pin 41 through an Au wire 43. The Au layer 8c
of the second electrode 8 and the lead pin 42 are connected to each
other through an Au wire 44.
Fabrication of the light-emitting diode as set out hereinabove is
described with reference to FIGS. 3 to 9.
The light-emitting diode 10 is fabricated by vapor phase growth
according to a metal organic vapor phase epitaxy technique
(hereinafter referred to MOVPE).
The gases used include NH.sub.3, an H.sub.2 carrier gas, trimethyl
gallium (Ga(CH.sub.3).sub.3) (hereinafter referred to simply as
TMG), trimethyl aluminum (Al(CH.sub.3).sub.3, hereinafter referred
to simply as TMA), silane (SiH.sub.4) and diethyl zinc (hereinafter
referred to simply as DEZ).
A single crystal sapphire substrate 1 having a surface oriented to
the direction (1120), i.e., "a"-surface, subjected to organic
washing and thermal treatment, is set on a susceptor which is
mounted in a reaction chamber of a MOVPE apparatus.
While passing H.sub.2 to the reaction chamber at a flow rate of 2
liters/minute at normal pressures, the sapphire substrate 1 is
subjected to vapor phase etching at 1200.degree. C. for 10
minutes.
Thereafter, the temperature is lowered to 400.degree. C., followed
by feeding H.sub.2 at 20 liters/minute, NH.sub.3 at 10
liters/minute and TMA at a rate of 1.8.times.10.sup.-5 moles/minute
to form a AlN buffer layer 2 with a thickness of 500 angstroms.
While keeping the sapphire substrate 1 at a temperature of
1150.degree. C., 20 liters/minute of H.sub.2, 10 liters/minute of
NH.sub.3 and 1.7.times.10.sup.-4 moles/minute of TMG are fed for 30
minutes to form a 2.5 .mu.m n-type layer 4 consisting of GaN having
a carrier concentration of 1.times.10.sup.15 /cm.sup.3.
The sapphire substrate 1 is then heated to 900.degree. C., followed
by feeding 20 liters/minute of H.sub.2, 10 liters/minute of
NH.sub.3, 1.7.times.10.sup.-4 moles/minute of TMG and
1.5.times.10.sup.-4 moles/minute of DEZ for two minutes, thereby
forming a 0.2 .mu.m thick i-type layer 5 made of GaN.
Thus, there is obtained a LED wafer having a multilayer structure
as shown in FIG. 3.
As shown in FIG. 4, a SiO.sub.2 layer 11 is formed in a thickness
of 1 .mu.m over the entire upper surface of i-type layer 5 by a
sputtering technique. A photoresist 12 is then formed on SiO.sub.2
layer 11, followed by photolithography to form an intended pattern
such that a portion of photoresist 12, corresponding to a portion
where a second electrode 8 is to be formed, is removed.
Thereafter, as shown in FIG. 5, the resultant exposed portion of
the SiO.sub.2 layer 11 is etched by means of a hydrofluoric acid
etchant through the mask of the photoresist 12.
As shown in FIG. 6, a recess 21 which reaches the n-type layer 4
through i-type layer 5 is formed by reactive ion etching through
the masks of the photoresist 12 and the SiO.sub.2 layer 11 while
feeding CCl.sub.2 F.sub.2 gas at a rate of 10 ml/minute under
conditions of a degree of vacuum of 0.04 Torr., and high frequency
power of 0.44 W/cm.sup.2. After completion of the etching, dry
etching with Ar is effected.
The photoresist 12 and the SiO layer 11 are removed by means of
hydrofluoric acid.
Subsequently, an approximately 1000 angstrom thick transparent
conductive ITO layer is formed over the entire surface by ion
plating. A photoresist is applied onto the ITO layer. The
photoresist is formed into a desired pattern by photolithography
while leaving the photoresist at a portion at which first electrode
7 is to be formed.
The exposed portion of the ITO layer is etched through the
photoresist mask. Thereafter, the photoresist is removed. By this
operation, the first electrode consisting of the ITO layer left
after the etching is formed as shown in FIG.7.
Subsequently, an Al layer is formed over the entire surface of the
sample in a thickness of approximately 2000 angstroms. A
photoresist is applied onto the Al layer, followed by
photolithography to form an intended pattern of the photoresist so
that a portion corresponding to second electrode 8 to be formed is
left.
The exposed portion of the Al layer is etched through the
photoresist mask, after which the photoresist is removed. By this
operation, an Al layer 8a which is used as second electrode 8 for
connection to the n-type layer 4 is formed as shown in FIG. 8.
A photoresist is applied over the entire upper surface of the
sample, followed by photolithography to remove the photoresist at
portions where the terminal electrode 9 for the first electrode 7
made of ITO and the second electrode 8 are to be formed,
respectively. As a result, a photoresist layer 31 is formed except
for the portions where the terminal electrode 9 and the second
electrode 8 are to be formed.
As shown in FIG. 9, a Ni layer 32 and an Au layer 33 are,
successively, formed over the entire upper surface of the sample in
thicknesses of about 500 angstroms and about 3000 angstroms,
respectively.
The photoresist 31 is removed by means of acetone to remove the Ni
layer 32 and the Au layer 33 formed on the photoresist 31, thereby
forming a Ni layer 9a and an Au layer 9b of the terminal electrode
9 for the first electrode 7 and a Ni layer 8b and an Au layer 8c
for the second electrode 8.
As shown in FIG. 1, Al is vacuum deposited on the entire opposite
side of the sapphire substrate 1 in a thickness of about 2000
angstroms to form a reflection film 13.
The resultant wafer is diced into individual chips. The LED chip is
fixed on a lead frame 40 as shown in FIG. 2. The lead pin 41 and
the Au layer 9b of the terminal electrode 9 for the first electrode
7 are connected by Au wire 43. The lead pin 42 and the Au layer 8c
of the second electrode 8 are connected by Au wire 44.
In this manner, a light-emitting diode having a MIS
(metal-insulator-semiconductor) structure can be fabricated.
When a voltage is applied such that the first transparent
conductive electrode 7 becomes positive in potential relative to
the second electrode 8, light is emitted at i-type layer 5 provided
beneath first electrode 7. The light can be directly picked up
through first transparent electrode 7. Moreover, the light
reflected from the reflection film 13 formed on the opposite side
of the sapphire substrate 1 is obtained through first transparent
electrode 7.
This light emitting diode makes use of a transparent conductive
film as the first electrode 7. Thus, the area of the first
electrode 7 is enlarged. This makes a small series resistance
between the first electrode 7 and the second electrode 8, thereby
suppressing generation of heat.
This reflects on the current-voltage characteristic in which the
threshold Voltage at a current of 10 mA is 6 volts. With a
light-emitting diode having a known structure (i.e. LED using an
aluminum electrode as the first electrode), the threshold voltage
at a current of 10 mA is 8 volts. Thus, the threshold voltage is
reduced to about 3/4 of that in conventional diodes, thus lowering
the drive voltage.
In the light emitting diode 10 of the above embodiment, n-type
layer 4 has a single-layer structure. As shown in FIG. 10, a
light-emitting diode 10a may have a double-layer n-type structure
which includes a 1.5 .mu.m thick lower carrier concentration n-type
layer 4a connected to the i-type layer 5 and a 2.2 .mu.m thick high
carrier concentration n.sup.+ -type layer 3.
In this light-emitting diode 10a, an electric current passes
through the high carrier concentration n+ type layer 3 in a
horizontal direction. Thus, the resistance between electrodes can
be further reduced.
The high carrier concentration n.sup.+ -type layer 3 is formed by
keeping the temperature of the sapphire substrate at 1150.degree.
C. and feeding 20 liters/minute of H.sub.2, 10 liters/minute of
NH.sub.3, 1.7.times.10.sup.-4 moles/minute of TMG, and 200
ml/minute of silane (SiH.sub.4) diluted with H.sub.2 to 0.86 ppm
for 30 minutes thereby providing a film with a thickness of 2.2
.mu.m and a carrier concentration of 1.5.times.10.sup.18
/cm.sup.3.
A further embodiment is shown in FIG. 11, wherein a light-emitting
diode 10b includes a first electrode 7 which is provided at the
center of the chip and made of a transparent conductive film and a
second electrode 8 provided around the first electrode 7 and
connected to n.sup.+ -type layer 3.
In this arrangement, an Al layer which is the lowermost layer of
the second electrode 8 may be provided as a reflection layer,
resulting in an improvement of light emission efficiency.
The light-emitting diode 10b can be fabricated by the steps shown
in FIGS. 12-15.
As shown in FIG. 12, a AlN buffer layer 2, a high carrier
concentration n.sup.+ -type layer 3, a low carrier concentration
n-type layer 4a and an i-type layer 5 are successively formed on a
sapphire substrate 1 according to the procedure set out
hereinabove.
As shown in FIG. 13, the resultant multi-layered wafer is diced by
the use of a thick blade having, for example, a thickness of 250
.mu.m, and cross cut to an extent reaching the upper surface of the
sapphire substrate 1 from the i-type layer 5 through the lower
carrier concentration n-type layer 4a, high carrier concentration
n.sup.+ -type layer 3 and buffer layer 2.
In the same manner as in FIGS. 7 and 8, a first electrode 7
consisting of ITO and a second electrode 8a are formed as shown in
FIG. 14.
According to the procedure shown in FIG.9, Ni layer 9a and Au layer
9b of the terminal electrode 9, and Ni layer 8b and Au layer 8c of
the second electrode 2 are formed as shown in FIG. 15.
As shown in FIG. 15, the wafer is diced by means of a thin blade
having a thickness, for example, of 150 .mu.m to cut off the
sapphire substrate 1 into pieces at the half-cut portions where the
second electrode 8 has been cross cut.
In this manner a light-emitting diode 10b having such a structure
as shown in FIG. 11 is fabricated.
Further, as shown in FIG. 16, a light-emitting diode 10c may be
fabricated as follows: a small-size hole which extends to the
n.sup.+ -type is formed at a central portion of i-type layer 5, and
a second electrode 8 is formed in the hole, about which a first
transparent conductive electrode 7 is formed.
In light-emitting diodes 10b, 10c having such structures as stated
hereinabove, second electrode 8 for the high carrier concentration
n.sup.+ -type layer 3 has a symmetric positional relation with
first electrode 7 for i-type layer 5.
Accordingly, the electric current passing between these electrodes
is substantially uniform irrespective of the position of the i-type
layer 5. Accordingly, uniform light emission in the blue
light-emitting region of the diodes is ensured with an improved
light emission intensity.
While this invention has been described in connection with what is
presently considered to be the most practical and preferred
embodiments, it is to be understood that the invention is not
limited to the disclosed embodiments, but, on the contrary, is
intended to cover various modifications and equivalent arrangements
included within the spirit and scope of the appended claims.
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